Adhesion promoters and methods of their synthesis and use

ABSTRACT

The invention relates to a new class of alkoxy silane compounds of the following general formula: ##STR1## wherein R is aliphatic or aromatic having from 1 to about 15 carbon atoms and R&#39;comprises a hydrolyzable alkoxy functionality. These compounds are particularly useful as adhesion promoters when used with adhesives such as, for example, phenylethynyl terminated polyimide or polysulfide adhesives. The invention also relates to a method of synthesizing the alkoxy silane compounds defined above.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. pat. application ofCrook, et al., Ser. No. 08/327,305, filed Oct. 21, 1994 and entitled"Method of Surface Preparation of Aluminum Substrates now U.S. Pat. No.5,520,768;" U.S. pat. application of Crook, et al., Ser. No. 08/550,957,filed Oct. 31, 1995 and entitled "Method of Surface Treatment ofTitanium Substrates now U.S. Pat. No. 5,660, 884; " and United StatesProvisional Patent Application of Crook, et al., Ser. No. 60/011,884,filed Feb. 20, 1996, and entitled "Method of Surface Preparation ofMetal Substrates and For Synthesis of Adhesive Promoters;" whichapplications are incorporated herein by this reference.

BACKGROUND

1. The Field of the Invention

The present invention is related to surface cleaning and preparation ofmetal substrates in order to provide improved bonding to thosesubstrates. The present invention is further related to a new class ofadhesion promoters, including specifically alkoxy silanes, and methodsof synthesis of those materials.

2. Technical Background

In constructing various structures from metals it is important to havethe capability of bonding to metal surfaces. This includes bonding metalsurfaces to other metal surfaces, as well as bonding non-metal materialsto metal surfaces. In many applications it is possible to use simplemechanical bonding mechanisms, such as bolts, screws, or rivets. Inother applications, concerns over the added weight of mechanicalfasteners make the use of adhesive more viable. Various adhesives areknown and commonly used in the art of bonding metals together or bondingnon-metal materials to metals. For example, various epoxy-basedadhesives are widely used for these applications.

When metals are bonded using an adhesive it is generally important toprovide the strongest possible bond. In the past it was difficult toassure a strong bond when using adhesive. For example, processingconditions during bond fabrication often cause dramatic reductions inbond strength.

Metal and metal alloys such as titanium, titanium alloys, steel, andsteel alloys are generally considered difficult to bond to. Some metals,for example, have a propensity to form a weak hydrated surface layer ofmetal oxide. With these metals the surface morphology and hence,adhesive bond durability, is dependent upon the type of surfacetreatment received prior to bonding.

A widely used treatment for metal surfaces involves vapor degreasing andgrit blasting. However, these methods do not provide sufficient initialbond strength or bond durability for adhesives such as phenylethynylterminated polyimides as evidenced by significant metal interfacialfailure. Therefore, the bondline properties are very sensitive to therelative humidity and process times from surface preparation to bonding.Moisture can also penetrate the bondline during aging, resulting indegradation of bondline properties and interfacial failures.

Available cleaning solvents used in this process have become morerestrictive because of environmental regulation on chemical wastedisposal. Thus, the combination of process sensitivity, marginal bonddurability, and environmental constraints, raises concerns overcontinued use of traditional surface treatment processes.

There are limited alternative methods of metal surface treatment andpreparation. Some of these other methods of metal surface preparationinvolve formation of stable, moisture-resistant oxide layers. Thesemethods include sulfuric, chromic, and phosphoric acid anodization.These electrolytic processes inhibit the further growth of corrosionsurface layers and enhance initial bond strength and bond durability. Inaddition, the phosphoric acid process produces a honeycomb surface whichis believed to enhance bond strength through mechanical interlocking.

These processes, however, generally consist of a complex series oftreatments including degreasing, alkaline cleaning, acid etching, acidanodization, and in some instances, a post treatment process includingprimers and coatings. These processes use heavy metals, acids, causticsand other hazardous and toxic chemicals which pose handling and disposalproblems. Clearly, environmental constraints limit the usefulness ofthese traditional surface treatment processes.

Accordingly, what is needed in the art are effective and efficientmethods of surface preparation and treatment of titanium, steel, andother metals, in order to provide stable adhesive bonding to metalsubstrates. In that regard it would be a significant advancement in theart to provide methods of surface treatment and preparation which wererelatively simple, and which used readily available materials. It wouldbe a related advancement in the art to provide such methods whichemployed materials that did not present a significant environmentalhazard. It would be an advancement in the art to provide improvedmaterials which promote adhesion. It has been discovered, as discussedbelow, that materials such as 4-phenylethynyl-N [3-triethoxysilylpropyl]phthalimide can be employed to promote adhesion. It would also be asignificant advancement in the art to provide improvement methods ofsynthesizing this material for use as an adhesion promoter.

Such methods are disclosed and claimed herein.

BRIEF SUMMARY AND OBJECTS OF THE INVENTION

The present invention relates to a new class of compounds that areparticularly adaptable for use as adhesion promoters. These materialsinclude a new group of phenylethynyl terminated silanes. The presentinvention is also related to methods of synthesizing these compounds.These materials are useful, upon hydrolysis of the ethoxyfunctionalities, for adhesion promotion when applied to metal surfaceswith, for example, phenylethynyl terminated polyimide or polysulfideadhesives.

In one aspect, the present invention is related to the preparation andsynthesis of the phenylethynyl terminated silanes of the presentinvention. One example of such a material is 4-phenylethynyl-N[3-triethoxysilylpropyl] phthalimide. Because of the highly conjugatedstructure of the compound it fluoresces, making its detection andinspection on the metal surface easy. The material is a clear yellowviscous liquid.

These materials are synthesized and used by employing a two-stepprocedure. In the case of the specific material mentioned above, thefirst step involves the reaction of 4 phenylethynyl phthalic anhydridewith 3 aminopropyl triethoxy silane to yield 4-phenylethynyl-N[3-triethoxysilylpropyl] phthalimide propyltriethoxy (PEPI). The secondstep hydrolyses the ethoxy groups enabling the silane to react with themetal oxides on the bond surface.

These effective adhesion promoters are useful with a number ofadhesives. For example, materials within the scope of the presentinvention have been synthesized for use as adhesion promoters for aphenylethynyl terminated polyimide adhesive (commercially designated"LaRC-PETI V"). This resin has been incorporated into a commerciallyavailable film adhesive known as FMX-5 manufactured by CYTEC Industries.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an IR spectrum of one composition within the scope of thepresent invention.

FIG. 2 is a proton NMR spectrum of the compound analyzed in FIG. 1.

FIG. 3 is a ¹³ C NMR spectrum of the compound analyzed in FIG. 1.

FIG. 4 is an IR spectrum of one composition within the scope of thepresent invention.

FIG. 5 is a is a ¹³ C NMR spectrum of the compound analyzed in FIG. 4.

FIG. 6 is a is a proton NMR spectrum of the compound analyzed in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As mentioned above, the present invention is related to the synthesis ofeffective adhesion promoters. The materials within the scope of theinvention comprise trialkoxysilyl compositions having the followinggeneral structure: ##STR2## wherein R is aliphatic or aromatic havingfrom 1 to 15 carbon atoms and R' is a hydrolyzable alkoxy functionality.In most applications, R will preferably contain 2 to 8 carbon atoms. Onespecific example of the compositions of the present invention is4-phenylethynyl-N [3-triethoxysilylpropyl] phthalimide wherein R is apropyl group.

The compositions within the scope of the present invention are prepareby a simple, but novel, synthesis. To prepare the compositions 4phenylethynyl phthalic anhydride is reacted with the selectedaminoalkoxy silane. One example of the a synthesis within the scope ofthe present invention is as follows: ##STR3##

As mentioned above, these materials are useful compounds for adhesionpromotion when applied to metal surfaces with, for example,phenylethynyl terminated polyimide and polysulfide adhesives.

Examples

The following examples are given to illustrate various embodiments whichhave been made or may be made in accordance with the present invention.These examples are given by way of example only, and it is to beunderstood that the following examples are not comprehensive orexhaustive of the many types of embodiments of the present inventionwhich can be prepared in accordance with the present invention.

Example 1

4-phenylethynyl-N [3-triethoxysilylpropyl] phthalimide was synthesizedas follows. 4-phenylethynyl phthalic anhydride (PEPA) was added to around bottom flask containing sufficient tetrahydrafuran (THF) todissolve the PEPA. An equimolar amount of 3-aminopropytriethoxy silane(3-APS) was added to the mixture at room temperature. The mixture wasthen heated over a steam bath until the THF mixture began to boil (T=65°C.). The heating continued for approximately 5 minutes. The solution wasthen removed from the steam bath and allowed to cool. The solvent wasremoved using a rotovaporator at 40° C. The resulting product is athick, viscous, yellow (amber) liquid.

Example 2

The compound synthesized accordingly to Example 1 was characterizedusing IR spectra. The IR spectra confirm the formation of the imide bythe dual carbonyl stretching frequencies at 1787 and 1729 cm⁻¹ as shownin FIG. 1. The peak of 2220 cm⁻¹ is assigned to carbon triple bondcarbon stretch. A small peak at 1850 cm⁻¹ has been assigned to thephthalic anhydride carbonyl stretching frequency. Proton NMR show nosign of hydrolyzed ethoxy functionality, as shown in FIG. 2, while the¹³ C NMR show evidence of --C.tbd.C--adjacent to phenyl at 87 and 92 ppmand carbon ortho to --C.tbd.C--at 122 ppm as illustrated in FIG. 3. The¹³ C also shows carbonyl adjacent to phenyl at 167 and 169 ppm. The twopeaks may reflect the presence of both the amic acid and the imide.

Example 3

Next, a sodium acetate/acetic acid buffer system was created by adding11g sodium acetate trihydrate to distilled water followed by 2milliliters of glacial acetic acid. The solution was diluted to 1 liter.This gives a pH of 5.0.

The PEPI silane was dissolved in THF and buffer solution was added togive a ratio of THF/buffer/PEPI silane of 13/6/1. If the mixture becamecloudy, a small amount of the THF was added to dissolve the precipitate.The THF/buffer/PEPI solution was extracted in methyl chloroform toremove unreacted 4-PEPA and 3-APS. The THF phase was recovered andisolated using a rotovaporator at 40° C. for both 2 hours and ambientfor 4 hours. The product recovered was a yellow crystalline powder. Themelting point of the solid was approximately 152° C. This material isthe polymeric form of the silane.

IR spectra show the presence of the preservation of both the ethynyl andimide functionalities. Comparison of the experimental material with amodel aliphatic imide (N-(4-bromobutyl) phthalimide show significantsimilarities (see FIG. 4). Some evidence exists for the presence of theamic acid. The ¹³ C NMR also shows the preservation of the imide andthen ethynyl functionality (see FIG. 5). Proton NMR shows the presenceof the --OH functionality at 4.1 ppm confirming the hydrolysis of theethoxy functionality (see FIG. 6).

Example 4

The material prepared according to Examples 1-4 is an adhesion promoter.In addition, because of the extensive conjugated multiple bonds, thecoating created by this material fluoresces with great efficiency,making its detection on the surface easy (FIG. 7.)

The material has been synthesized for use as an adhesion promoter for aphenylethynyl terminated polyimide adhesive (designated "LaRC-PETI V").LaRC-PETI 5 is now manufactured as an experimental firm adhesive byCytec Industries (manufactured as FMX-5).

Titanium lap shear panels were treated by first grit blasting toaccomplish a vapor degreasing. The panels were then treated with a 0.2%sodium metasilicate solution, followed by 5% (wt./wt.) phenylethynylterminated silane primer. The treated panels were then bond tested. Allof the panels tested at room temperature failed at 3299 psi (5%solution) and 100% interfacial failure mode. Next a polyamic acidsolution dissolved in N-methyl pyrrolidone (NMP) was added to thephenylethynyl terminated primed titanium lap shear panels, and thenbonded. The results demonstrated a dramatic increase in the lap shearstrength. These panels failed at an average lap shear strength of 7314psi at room temperature and 4452 psi at 177° C. The failure mode waspredominantly cohesive in the adhesive.

Further tests were performed to examine the relative contribution tobonding of the silane primer relative to that of the amic acid solution(one commercially available polyamic acid solution is designated BRX-5).Sample set 1 and 2 were vapor degreased and grit blasted, followed by arinse with a 0.2% sodium metasilicate solution. Only sample set 2 wastreated with a phenylethynyl terminated silane primer. Both sample setswere treated with the polyamic acid solution prior to bonding. Sampleset 1 samples had an average lap shear strength of 4162 psi at roomtemperature and 3629 psi at 177° C. The average lap shear strength ofsamples in sample set 2 was 6793 psi at room temperature and 4264 psi at177° C.

The data clearly indicates that the phenylethynyl terminated silaneprimer is playing a significant role in enhancing the adhesion ofLaRC-PETI V to titanium and the hypothesis that 4-phenyethynyl-N[3-triethoxysilylpropyl] phthalimide propyl ethoxy silane solution is anadhesion promoter.

Summary

In summary the present invention provides improved methods of promotingadhesion to titanium, steel, and other metals. The present inventionpromotes adhesion with materials that do not present a recognized andregulated environmental hazard. The present invention also provides asimple yet effective method of synthesizing the compounds of the presentinvention.

The invention may be embodied in other specific forms without departingfrom its spirit or essential characteristics. The described embodimentsare to be considered in all respects only as illustrative and notrestrictive. The scope of the invention is, therefore, indicated by theappended claims rather than by the foregoing description. All changeswhich come within the meaning and range of equivalency of the claims areto be embraced within their scope.

What is claimed and desired to be secured by Letters Patent is:
 1. Asilane composition comprising: ##STR4## wherein R is aliphatic oraromatic having from 1 to about 15 carbon atoms and R' comprises ahydrolyzable alkoxy functionality.
 2. A composition as defined in claim1 wherein R comprises 3 carbon atoms.
 3. A composition as defined inclaim 1 wherein the composition comprises 4-phenylethynyl-N[3-triethoxysilylpropyl] phthalimide.
 4. A composition as defined inclaim 1 wherein R is aliphatic or aromatic having from 2 to 8 carbonatoms.
 5. A composition as defined in claim 1 wherein R' comprises(OEt)₃. ##STR5##
 6. A method of synthesizing a composition having theformula wherein R is aliphatic or aromatic having from 1 to about 15carbon atoms and R'comprises a hydrolyzable alkoxy functionality, themethod comprising the step of reacting an amino triethoxy silane havingthe formula H₂ N-R-Si(R') with 4 phenylethynyl phthalic anhydride(PEPA).
 7. A method as defined in claim 6 wherein R comprises 3 carbonatoms.
 8. A method as defined in claim 6 wherein the amino triethoxysilane comprises 3 aminopropyl triethoxy silane.
 9. A method as definedin claim 6 wherein R is aliphatic or aromatic having from 2 to 8 carbonatoms.
 10. A method as defined in claim 6 wherein R' comprises (Oet)₃.